1. Field of the Invention
The present invention relates to a method for controlling rotational speed of an internal combustion engine in an isochronous control mode using a proportional and integral (PI) control.
2. Prior Art
For example, in a vehicle powered by an internal combustion engine equipped with an electronically controlled automatic transmission system in which the operations of a gear-type transmission and a clutch are controlled by means of a microcomputer, for shifting the gear-type transmission into a desired gear position it is necessary to synchronize rotational speeds between two gears to be meshed in the transmission (Japanese Patent Application Public Disclosure No. Sho 60-179365). To establish such a synchronization between the gears, an isochronous control is usually employed, and a fuel quantity to be supplied to an internal combustion engine is controlled by the use of PI control in order to make an actual rotational speed of a gear coincident with a desired target rotational speed thereof.
However, when the rotational speed of the internal combustion engine is controlled in PI control mode, it tends to degrade the response characteristics of the control for the variation of the target rotational speed. In this case, the improvement in the response characteristics will impair control stability. Furthermore, an overshoot or undershoot condition in engine rotational speed may be caused when the target rotational speed varies greatly for a short time. Therefore, it is generally said that PI control is unsuitable where the target rotational speed differs greatly from the actual rotational speed and it is required to make the actual rotational speed equal to the target rotational speed for a short time, for example in the above example of the synchronization control of the gear rotational speed for the gear-shifting operation of the above-mentioned transmission system.